In light of the goal to establish a sustainable pathway to polymers, exploitation of renewable resources through green production procedures is currently of high interest. Considering the depletion of petroleum resources and the accumulation of polymer waste, research efforts have focused on developing novel polymers and polymer production processes that use renewable feedstock.
Polyethylene is the largest volume synthetic polymer produced worldwide. It is produced in various forms and has acquired a large commercial market in films, sheets, pipes, fibres, containers, bottles, and many other applications. Its conventional production relies on petroleum based ethylene gas. Depending on the polymerization process, low or high density polyethylene can be produced under high or low pressure, respectively. Moreover, the catalyst used for the production process has an effect on the macromolecular architecture of the formed polyethylene. A still important procedure for the production of polyethylene was introduced by Philips using a chromium based catalyst (Cr/SiO2). Although this system has poor control on the molecular weight, it still covers the major volume of the global polyethylene production. In the mid 1950's, with the discovery of Ziegler-Natta catalysts, and later on the metallocene and other single-site catalysts, it was possible to achieve higher control on polydispersity indices, which are about 4 and 2, respectively. For all mentioned processes fully petroleum derived ethylene is being used in order to synthesize large amounts of this highly important polymer. Due to the awareness of oil depletion and the waste problem as well as legislation, there is a need for alternative production processes for polyethylene wherein renewable feedstock is used.
Alternatively, so-called polyethylene mimics are explored. These are polymers that have polyethylene-like properties and behavior. For example, polyesters can form a biodegradable alternative for polyethylene in many applications. Polyesters are of high commercial interest because of the properties that they can exhibit. These properties, for instance, include biocompatibility, biodegradability and drug permeability. Therefore, polyesters are of great interest, e.g., for medical and food packaging applications. For these purposes, materials with an engineered structure are desired, which implies the need for a high level of control over the polymerization reaction.
The synthesis of polyesters with polyethylene-like properties via ring-opening polymerization of a renewable macrolactone has been reported (Van der Meulen et al. Macromolecules 2011, 44 (11), 4301-4305). Due to the presence of ester functionalities within the monomers reported therein, the prepared polymers can favorably be considered as biodegradable, thus differing considerably from polyethylene. Also, polycondensation of stoichiometric amounts of dimethyl 1,26-hexacosanedioate and 1,26-hexacosanediol, with titanium(IV)butoxide as a catalyst precursor to afford a polyester-26,26 has been described (Stempfle et al. Macromolecular Rapid Communications 2013, 34, 47-50).
These known polyethylene mimics, however, have a relatively high number of ester groups within the polymer chain, while polyesters having long methylene sequences would mimic polyethylene properties better. As the structure of the final product is restricted by the synthesis route, the versatility of fine-tuning the material's properties is limited.